U.S. patent number 6,719,948 [Application Number 09/863,073] was granted by the patent office on 2004-04-13 for techniques for infiltration of a powder metal skeleton by a similar alloy with melting point depressed.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Samuel M. Allen, Adam M. Lorenz, Emanuel M. Sachs.
United States Patent |
6,719,948 |
Lorenz , et al. |
April 13, 2004 |
Techniques for infiltration of a powder metal skeleton by a similar
alloy with melting point depressed
Abstract
In infiltrating a porous metal skeleton an infiltrant
composition is used similar to that of the powder skeleton, but
with the addition of a melting point depressant. The infiltrant
quickly fills the skeleton. As the melting point depressant
diffuses into the base powder, the liquid may undergo diffusional
solidification and the material eventually homogenizes. Maintaining
the infiltrant at a liquidus composition for the infiltration
temperature typically ensures that the bulk composition or
properties will remain uniform throughout the part, particularly in
the direction of infiltration. Success of such an infiltration is
enhanced by effective means of maintaining the molten infiltrant at
a liquidus composition. It is also beneficial, in some cases, for
the time scale of the infiltration to be much faster than the time
scale of the diffusion of the melting point depressant and the
subsequent solidification and homogenization. The relative rates of
infiltration and diffusion/solidification rate are significantly
impacted by the choice of materials system. Other factors also
influence these rates. They include: selection of powder size
(diameter), shape, surface roughness, and size distribution,
feeding liquid from different locations, liquid feeder channels,
smoothing of the part surface with fine powder and affecting
infiltrant fluid properties. Various material systems are also
disclosed, as are methods of designing a process of infiltrating a
part, including binary and ternary and higher component systems.
Homogeneous composition may be achieved using these techniques,
particularly along the direction of infiltration.
Inventors: |
Lorenz; Adam M. (Somerville,
MA), Sachs; Emanuel M. (Newton, MA), Allen; Samuel M.
(Jamaica Plain, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
25340164 |
Appl.
No.: |
09/863,073 |
Filed: |
May 21, 2001 |
Current U.S.
Class: |
419/27 |
Current CPC
Class: |
B22F
3/26 (20130101); B33Y 80/00 (20141201) |
Current International
Class: |
B22F
3/26 (20060101); B22F 003/26 () |
Field of
Search: |
;419/27 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Banerjee, S., Oberacker, R., and Goetzel, C., "Experimental Study
of Capillary Force Induced Infiltration of Compacted Iron Powders
with Cast Iron," Modern Developments in Powder Metallurgy, vol. 16,
Metal Powder Industries Federation: Princeton, NJ, pp. 209-244,
1984. .
Carman, C., Flow of gases through porous media. Butterworths:
London, pp. 8-13, 1956. .
Messner, R. and Chiang, Y., "Liquid-Phase Reaction-Bonding of
Silicon Carbide Using Alloyed Silicon-Molybdenum Melts," Journal of
the American Ceramic Society, vol. 73, No. 5, pp. 1193-1200, 1990.
.
Scherer, G., "Theory of Drying," Journal of the American Ceramic
Society, vol. 73, No. 1, pp. 3-14, 1990. .
Sercombe, T., Loretto M., and Wu, X., "The Production of Improved
Rapid Tooling Materials, " Advances in Powder Metallurgy and
Particulate Materials, pp. 3-25 to 3-36, Proceedings of the 2000
International Conference of Powder Metallurgy and Particulate
Materials, May 30-Jun. 3, 2000. Metal Powder Industries Federation:
Princeton, NJ. .
Tanzilli, R. and Heckel, R., "Numerical Solutions to the Finite,
Diffusion-Controlled, Two-Phase, Moving-Interface Problem (with
Planar, Cylindrical, and Spherical Interfaces)," Transactions of
the Metallurgical Society of AIME, vol. 242, pp. 2313-2321, Nov.
1968. .
Thorsen, K., Hansen, S., and Kjaergaard, O., "Infiltration of
Sintered Steel with a Near-Eutectic Fe-C-P Alloy," Powder
Metallurgy International, Vol 15, No. 2, pp. 91-93, 1983. .
Zhuang, H., Chen, J., and Lugscheider, E., "Wide gap brazing of
stainless steel with nickel-base brazing alloys," Welding in the
World, vol. 24, No. 9/10, pp. 200-208, 1986. .
Zhuang, W. and Eagar, T., "Liquid infiltrated powder interlayer
bonding: a process for large gap joining," Science and Technology
of Welding and Joining, vol. 5, No. 3, pp. 125-135, 2000. .
Goetzel, Claus G., "Infiltration," ASM Handbook, vol. 7, Powder
Metallurgy, pp. 551-566, 1984. .
Landford, George, "High Speed Steel made by Liquid Infiltration,"
Materials Science and Engineering, 28, pp. 275-284, 1977. .
Langford, George and Cunningham, Robert E., "Steel Casting by
Diffusion Solidification", Metallurgical Transactions B, vol. 9B,
pp. 5-19, Mar. 1978..
|
Primary Examiner: Jenkins; Daniel
Attorney, Agent or Firm: Weissburg; Steven J.
Government Interests
GOVERNMENT RIGHTS
The United States Government has certain rights in this invention
pursuant to the Office of Naval Research Award #N0014-99-1-1090,
Research in Manufacturing and Affordability, awarded on Sep. 30,
1999.
Parent Case Text
PRIORITY CLAIM
This claims priority to U.S. Provisional application No.
60/206,066, filed on May 22, 2000, the full disclosure of which is
fully incorporated by reference herein.
Claims
Having described the inventions, what is claimed is:
1. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder composed of at least two elements; b.
providing an infiltrant comprising: i. the same at least two
elements as are in the skeleton; and ii. melting point depressant
(MPD); the infiltrant having a composition that is a liquidus
composition for an infiltration temperature; and c. infiltrating
said skeleton with said infiltrant, said infiltration driven
primarily by capillarity at approximately said infiltration
temperature.
2. The method of claim 1, further comprising the step of subjecting
said infiltrated skeleton to conditions such that at least some of
said MPD diffuses from said infiltrated porosities into said metal
powder, and diffusional solidification occurs.
3. The method of claim 1, said step of providing infiltrant
comprising: providing, in a vessel, an infiltrant supply having a
bulk composition within a multi-phase field where at said
infiltration temperature solid is present and liquid is present at
a liquidus composition, and further comprising the steps of: a.
melting a portion of said infiltrant supply; and b. agitating said
melted portion of said infiltrant supply throughout its volume, to
a degree that ensures that said liquid remains at said liquidus
composition.
4. The method of claim 3, said step of agitating comprising
stirring said melted portion of said infiltrant supply.
5. The method of claim 3, said step of agitating comprising
bubbling gas through said melted infiltrant supply.
6. The method of claim 3, said step of agitating comprising shaking
said melted infiltrant supply.
7. The method of claim 3, said step of agitating comprising
applying an electromagnetic inductive field to said melted
infiltrant precursor supply.
8. The method of claim 3, said step of agitating comprising tipping
said vessel back and forth.
9. The method of claim 2, said step of subjecting said infiltrated
skeleton to temperature conditions such that diffusional
solidification occurs, comprising subjecting said skeleton to a
temperature range that exceeds said infiltration temperature.
10. The method of claim 9, said step of subjecting said infiltrated
skeleton to a temperature range that exceeds said infiltration
temperature comprising maintaining said infiltrated skeleton at
substantially constant temperature, such that solidification occurs
substantially isothermally.
11. The method of claim 1, said step of infiltrating said
porosities of said skeleton with said melted infiltrant comprising
substantially fully filling substantially all of said network of
interconnected porosities with said melted infiltrant.
12. The method of claim 2, said step of subjecting said infiltrated
skeleton to temperature conditions such that said MPD diffuses
comprising subjecting said infiltrated skeleton to temperature
conditions such that said MPD diffuses from said infiltrated
network of porosities into and substantially throughout said metal
powder.
13. The method of claim 1, a. wherein said step of providing
infiltrant comprises providing, in a vessel, an infiltrant supply,
having a bulk composition that is in an equilibrium multiple phase
field at said infiltration temperature, such that solid is present
and liquid is present at a liquidus composition; and further
comprising the step of: b. overheating said infiltrant supply to a
temperature that exceeds said infiltration temperature and
maintaining said overheating such that at least some of said
infiltrant that is solid in said multiple phase field becomes
liquid.
14. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder composed of two or more elements,
chosen as in step e below; b. providing an infiltrant comprising:
i. the same elements as are in the skeleton; and ii. melting point
depressant; the infiltrant having a composition that is a liquidus
composition for an infiltration temperature, the liquidus
composition and infiltration temperature chosen as in step e below;
c. infiltrating said skeleton at said infiltration temperature with
said infiltrant in liquid form, said infiltration driven primarily
by capillarity; d. subjecting said infiltrated skeleton to
conditions such that a portion of said melting point depressant
diffuses from said infiltrated porosities into said metal powder,
and at least partial diffusional solidification occurs; and e.
choosing said metal powder composition, melting point depressant,
infiltrant composition and infiltration temperature such that
during diffusional solidification of said infiltrant, relative
ratios, of components other than melting point depressant, in said
liquid infiltrant not yet solidified, remain constant.
15. The method of claim 14, said melting point depressant
consisting essentially of a single element.
16. The method of claim 14, said melting point depressant
consisting essentially of two or more elements, all of which have
similar mass transport characteristics relative to said elements of
said skeleton.
17. The method of claim 14, said step of subjecting said
infiltrated skeleton to conditions such that at least partial
diffusional solidification occurs comprising subjecting said
infiltrated skeleton to constant temperature conditions such that
at least partial isothermal solidification occurs.
18. The method of claim 14, said step of subjecting said
infiltrated skeleton to conditions such that at least partial
diffusional solidification occurs comprising subjecting said
infiltrated skeleton to reducing temperature conditions.
19. The method of claim 14, said skeleton further comprising
melting point depressant.
20. The method of claim 14, said skeleton being substantially free
of melting point depressant.
21. The method of claim 14, said step of choosing comprising
choosing said metal powder composition, melting point depressant,
infiltrant composition and infiltration temperature such that a
liquidus composition and a solidus composition of said infiltrant,
that are joined by a tie line on an equilibrium phase diagram, both
lie on a line of constant relative proportions of non-MPD
components of said infiltrant.
22. The method of claim 21, said step of choosing comprising
choosing said metal powder composition, melting point depressant,
infiltrant composition and infiltration temperature such that the
composition of said skeleton, lies on said line of constant
relative proportions of non-MPD components of said infiltrant.
23. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder comprising a single metal; b.
providing an infiltrant comprising: i. the same metal as is in the
skeleton; and ii. melting point depressant consisting essentially
of a single element; the infiltrant having a composition that is
the liquidus composition for an infiltration temperature; c.
infiltrating said skeleton at said infiltration temperature with
said infiltrant in liquid form, said infiltration driven primarily
by capillarity; d. subjecting said infiltrated skeleton to
conditions such that a portion of said melting point depressant
diffuses from said infiltrated porosities into said metal powder,
and at least partial diffusional solidification occurs.
24. The method of claim 23, said step of subjecting said
infiltrated skeleton to conditions such that at least partial
diffusional solidification occurs comprising subjecting said
infiltrated skeleton to constant temperature conditions such that
at least partial isothermal solidification occurs.
25. The method of claim 23, said step of subjecting said
infiltrated skeleton to conditions such that at least partial
diffusional solidification occurs comprising subjecting said
infiltrated skeleton to reducing temperature conditions.
26. The method of claim 23, said skeleton further comprising
melting point depressant.
27. The method of claim 23, said skeleton being free of melting
point depressant.
28. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said powder packed at a packing fraction, said metal
powder composed of at least one element, chosen as in step e below;
b. providing an infiltrant comprising: i. the same at least one
elements as are in the skeleton; and ii. melting point depressant;
the infiltrant having a composition that is a liquidus composition
for an infiltration temperature, the liquidus composition and
infiltration temperature chosen as in step e below; c. infiltrating
said skeleton at said infiltration temperature with said infiltrant
in liquid form, said infiltration driven primarily by capillarity;
d. subjecting said infiltrated skeleton to conditions such that a
portion of said melting point depressant diffuses from said
infiltrated porosities into said metal powder, and at least partial
diffusional solidification occurs; and e. choosing skeleton packing
fraction, said metal powder composition, melting point depressant,
infiltrant composition and infiltration temperature such that after
diffusional solidification of said infiltrant at said infiltration
temperature ceases, an interconnected network of liquid, remains
substantially throughout said skeleton.
29. The method of claim 28, further wherein said interconnected
network of liquid is sufficiently porous to permit flow of
infiltrant therethrough.
30. The method of claim 28, further comprising the step of
subjecting said infiltrated skeleton to lower temperature
conditions such that all of said infiltrant solidifies such that
said infiltrated skeleton achieves a bulk composition substantially
identical to that of a casting of said infiltrant.
31. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout a geometry, said metal powder composed of at least one
element, chosen as in step e below, with powder particle
composition and characteristics chosen as in step e below; b.
providing an infiltrant comprising: i. the same elements as are in
the skeleton; and ii. melting point depressant; the infiltrant
having a composition, chosen as in step e below; c. infiltrating
said skeleton at an infiltration temperature chosen as in step e
below, with said infiltrant in liquid form, said infiltration
driven primarily by capillarity; d. subjecting said infiltrated
skeleton to conditions such that after said step of infiltration
has substantially completed such that said skeleton geometry is
fully infiltrated, a portion of said melting point depressant
diffuses from said infiltrated porosities into said metal powder,
and at least partial diffusional solidification occurs; and e.
choosing said metal powder composition, melting point depressant,
infiltrant composition and infiltration temperature and metal
particle size, size distribution, and surface roughness, such that
said infiltrant infiltrates throughout substantially all of said
network of interconnected porosities before essentially any of said
diffusional solidification has occurred; whereby said infiltrated
skeleton is substantially free of compositional gradient along a
direction of infiltration.
32. The method of claim 31, said step of choosing, in the case of
infiltrating without opposing gravity, comprising choosing a
representative size of said powder material, and, then if the
resultant rate of infiltration is too slow to infiltrate
substantially all of said network before any diffusional
solidification occurs, choosing a relatively larger representative
size of powder particle to increase the rate of infiltration.
33. The method of claim 31, said step of choosing, in the case of
infiltrating against gravity, comprising choosing a representative
size of said powder material, and, then if the resultant rate of
infiltration is too slow to infiltrate substantially all of said
skeleton before any diffusional solidification occurs, choosing a
relatively larger representative size of powder particle to
increase the rate of infiltration, but limiting the choice of
relatively larger size particles to a particles small enough to
achieve a capillary driving force to overcome gravity to the full
height of said geometry.
34. The method of claim 31, said step of choosing, in the case of
infiltrating against gravity, having an acceleration g, in a
skeleton having a geometry with height h, comprising choosing: said
powder to have a surface area S.sub.p of the pore space in the
skeleton and a volume V.sub.p of the pore space in the skeleton;
said infiltrant to have a liquid density .rho., and liquid/vapor
interfacial energy .gamma..sub.LV and a contact angle .theta. with
the solid of the skeleton powder, such that: ##EQU7##
35. The method of claim 34, said step of choosing, further
comprising choosing substantially mono-modal spherical particles,
and choosing said skeleton to have a void fraction .epsilon. and
said spherical particles to having a diameter D, such that:
##EQU8##
36. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout a geometry, said metal powder composed of at least one
element, chosen as in step e below, with powder particle
composition and characteristics chosen as in step e below; b.
providing an infiltrant comprising: i. the same elements as are in
the skeleton; and ii. melting point depressant; the infiltrant
having a composition chosen as in step e below; c. infiltrating
said skeleton at an infiltration temperature, chosen as in step e
below, with said infiltrant in liquid form, said infiltration
driven primarily by capillarity; d. subjecting said infiltrated
skeleton to conditions such that after said step of infiltration
has been substantially completed such that said skeleton geometry
is fully infiltrated, a portion of said melting point depressant
diffuses from said infiltrated porosities into said metal powder,
and diffusional solidification occurs to an extent that blocks off
flow of infiltrant throughout said interconnected porosities; and
e. choosing said metal powder composition, said melting point
depressant, said infiltrant composition said infiltration
temperature, said metal particle size, and metal particle size
distribution, and surface roughness, such that said infiltrant
infiltrates throughout substantially all of said network of
interconnected porosities before diffusional solidification occurs
to an extent that blocks off flow of infiltrant throughout said
interconnected porosities.
37. A method for infiltrating a substantially metal part,
comprising the steps of: a. providing a skeleton having: i. an
interconnected adhered metal powder body having a network of
interconnected porosities throughout, said porosities having a
characteristic pore size; ii. at least one infiltrant contact
surface; and iii. at least one feeder channel having a
characteristic diameter d that is at least three times said pore
size, said feeder channel extending from said infiltrant contact
surface to a first internal region of said network of porosities;
b. providing, an infiltrant supply, said infiltrant comprising: i.
the same elements as are in the skeleton; and ii. melting point
depressant; c. subjecting said infiltrant supply to an infiltration
temperature under conditions that melt at least a portion of said
infiltrant supply; d. contacting said infiltrant contact surface of
said skeleton to said melted infiltrant supply, such that liquid
infiltrant passes through said feeder channel to said internal
region; e. subjecting said skeleton to conditions such that said
liquid infiltrant is driven primarily by capillarity and
infiltrates said interconnected porosities of said skeleton,
including said internal region; f. subjecting said infiltrated
skeleton to temperature conditions such that a portion of said
melting point depressant diffuses from said infiltrated porosities
into said metal powder; and g. subjecting said infiltrated skeleton
to temperature conditions such that infiltrant that has infiltrated
said porosities, solidifies.
38. The method of claim 37, said network of porosities having a
diffusional solidification related penetration distance limit (PL),
said skeleton having a geometry and dimension such that a second
internal region of said network of interconnected porosities is; a.
spaced from said infiltrant contact surface a distance that exceeds
said penetration distance limit; and b. spaced from said feeder
channel a distance that is less than said penetration distance
limit; whereby said infiltrant infiltrates said skeleton to said
second internal region through said feeder channel, beyond said
penetration distance limit from said infiltrant contact
surface.
39. The method of claim 37, said feeder channel having a diameter
of at least five times said characteristic pore size.
40. The method of claim 37, said skeleton having a geometry and
dimension from said infiltrant contact surface, such that
infiltrant must travel to a height h above said infiltrant contact
surface to reach said second internal region of said network of
porosities, said feeder channel having a radius r that is less
than: ##EQU9##
where .rho. is the density of said liquid infiltrant, g is
acceleration due to gravity, .gamma..sub.LV is the liquid/vapor
interfacial energy, and .theta. is the contact angle of the liquid
with the solid.
41. The method of claim 38, said skeleton having a geometry and
dimension from said infiltrant contact surface, such that
infiltrant must travel to a height z above said infiltrant contact
surface to reach said second internal region of said network of
porosities, said feeder channel having a radius r that is less
than: ##EQU10##
where .rho. is the density of said liquid infiltrant, g is
acceleration due to gravity, .gamma..sub.LV is the liquid/vapor
interfacial energy, and .theta. is the contact angle of the liquid
with the solid.
42. The method of claim 37, said feeder channel comprising a
channel having a characteristic diameter of between five and ten
times said characteristic pore size.
43. The method of claim 37, said skeleton comprising a feeder
channel having at least two portions, inclined relative to each
other.
44. The method of claim 37, said feeder channel having a
characteristic diameter that varies along its length.
45. The method of claim 37, said at least one feeder channel
comprising a network of feeder channels.
46. A method for infiltrating a substantially metal part,
comprising the steps of: a. providing a skeleton of interconnected
adhered metal first powder having a surface and a network of
interconnected porosities throughout, said powder having a
relatively larger characteristic particle size; b. substantially
covering said surface of said skeleton with a covering layer
comprising relatively fine metallic powder, said relatively fine
powder having a characteristic size that is significantly smaller
than said relatively larger characteristic size; c. providing, in a
vessel, an infiltrant supply, said infiltrant comprising: i. the
same elements as are in the first powder of said skeleton; and ii.
melting point depressant; d. subjecting said infiltrant supply to
an infiltration temperature, under conditions that melt a portion
of said infiltrant supply; e. contacting said skeleton to said
melted infiltrant supply, such that infiltrant is drawn into said
skeleton through said relatively larger metal powder, driven
primarily by capillary action; f. infiltrating said interconnected
porosities of said relatively larger metal powder with said melted
infiltrant and infiltrating said covering layer with said melted
infiltrant, via said interconnected porosities of said relatively
larger metal powder; g. subjecting said infiltrated skeleton to
temperature conditions such that a portion of said melting point
depressant diffuses from said infiltrated porosities into said
relatively larger metal powder; and h. subjecting said infiltrated
skeleton to temperature conditions such that infiltrant that has
infiltrated said porosities, solidifies.
47. The method of claim 46, further, wherein an interconnected
network of porosities within a body of interconnected adhered said
fine powder has a diffusional solidification related penetration
distance limit, said skeleton having a geometry and dimension such
that: a. a region of said fine covering layer is spaced from said
infiltrant contact surface a distance that exceeds said penetration
distance limit; and b. said covering layer has a thickness that is
less than said penetration distance limit; whereby said infiltrant
infiltrates said fine covering layer through said interconnected
porosities of said relatively larger metal powder, beyond said
penetration distance limit from said infiltrant contact
surface.
48. The method of claim 46, said covering layer comprising
relatively fine metallic powder having particle sizes that are
between approximately 1/10 and approximately 1/100 said relatively
larger characteristic particle size.
49. The method of claim 46, said step of applying a covering layer
comprising applying said layer to said surface of said skeleton to
create a skin of finer powder over said relatively larger metal
particles at said surface.
50. The method of claim 46, said step of applying a covering layer
comprising applying said covering layer to said surface of said
skeleton so that said covering layer penetrates into porosities
between said relatively larger metal powder, leaving particles of
said relatively larger metal powder at said surface with, at most,
a thin covering layer of said finer powder.
51. The method of claim 46, wherein: a. said covering layer
comprises a paste having a polymeric vehicle; and b. further
comprising the step of subjecting said paste covered skeleton to
temperature conditions such that said polymeric vehicle burns off
and is substantially eliminated from said paste and said relatively
fine powder particles are sintered into place.
52. A method for infiltrating a substantially metal part,
comprising the steps of: a. providing a skeleton of interconnected
adhered metal powder having a network of interconnected porosities
throughout, said powder particles having: i. a size and shape such
that, if smooth, said particles would have a nominal surface area;
and ii. a surface texture that gives rise to an actual surface area
that exceeds said nominal surface area by between approximately 25%
and 500% of said nominal surface area; b. providing, an infiltrant
supply comprising: i. the same elements as are in the skeleton; and
ii. melting point depressant; c. subjecting said infiltrant supply
to an infiltration temperature, under conditions that melt at least
a portion of said infiltrant supply; d. contacting said skeleton to
said melted infiltrant supply, such that liquid infiltrant is drawn
into said skeleton through said network of porosities, driven
primarily by capillary action; e. infiltrating said interconnected
porosities of said skeleton with said melted infiltrant. f.
subjecting said infiltrated skeleton to temperature conditions such
that a portion of said melting point depressant diffuses from said
infiltrated porosities into said metal powder; and g. subjecting
said infiltrated skeleton to temperature conditions such that
infiltrant that has infiltrated said porosities, solidifies;
wherein a comparison network of porosities identical to said
network, but for having a representative particle surface area
equal to said nominal surface area, having a diffusional
solidification related penetration distance limit (PL), said
skeleton having a geometry and dimension such that a region of said
network of interconnected porosities is spaced from said infiltrant
contact surface a distance that exceeds said penetration distance
limit of said comparison network; whereby said infiltrant
infiltrates said skeleton to said region of said network by
capillarity through said interconnected porosities of particles
having a surface area that exceeds said smooth surface area, beyond
said penetration distance limit, from said infiltrant contact
surface.
53. The method of claim 52, said skeleton of powder particles
comprising hydrometallurgically processed powder.
54. The method of claim 52, said powder particles comprising vapor
phase etched powder.
55. The method of claim 52, said powder particles comprising the
relatively large powder particles that are each coated with a layer
of powder particles that are smaller than said relatively large
particles, said smaller particles having a size between 1/1000 and
1/10 the size of said relatively large particles.
56. A method for infiltrating a substantially metal part,
comprising the steps of: a. providing a skeleton having: i. an
interconnected adhered metal powder body having a network of
interconnected porosities throughout; ii. at least two infiltrant
contact surfaces; and iii. for each infiltrant contact surface, at
least one infiltrant supply tab coupled to said respective
infiltrant contact surface; b. providing, an infiltrant supply
comprising: i. the same elements as are in the skeleton; and ii.
melting point depressant; c. subjecting said infiltrant supply to
an infiltration temperature under conditions that melt at least a
portion of said infiltrant supply; d. coupling each of said
infiltrant supply tabs to said melted portion of said infiltrant
supply, such that liquid infiltrant passes through said supply tab
to said respective infiltrant contact surface; e. subjecting said
skeleton to conditions such that said liquid infiltrant infiltrates
said interconnected porosities of said skeleton, including regions
adjacent said infiltrant contact surfaces, said infiltration driven
primarily by capillarity; f. subjecting said infiltrated skeleton
to temperature conditions such that a portion of said melting point
depressant diffuses from said infiltrated porosities into said
metal powder; and g. subjecting said infiltrated skeleton to
temperature conditions such that infiltrant that has infiltrated
said porosities, solidifies.
57. The method of claim 56, said fluid supply tabs comprising
hollow tubes.
58. The method of claim 56, said fluid supply tabs comprising tubes
that are integral with said skeleton.
59. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder composed of iron; b. providing an
infiltrant comprising: i. the same elements as are in the skeleton;
and ii. melting point depressant (MPD) comprising an alloy of iron,
carbon, manganese, silicon, chromium, nickel and molybdenum; c.
infiltrating said interconnected porosities with said infiltrant in
liquid form, said infiltration driven primarily by capillarity; d.
subjecting said infiltrated skeleton to temperature conditions such
that said melting point depressant diffuses from said infiltrated
voids into said metal powder; and e. subjecting said infiltrated
skeleton to temperature conditions such that infiltrant that has
infiltrated into said interconnected porosities, solidifies.
60. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder composed of iron; b. providing an
infiltrant comprising: i. the same elements as are in the skeleton;
and ii. melting point depressant (MPD) comprising an alloy of iron,
carbon, manganese, silicon, chromium, nickel, copper and niobium;
c. infiltrating said interconnected porosities with said infiltrant
in liquid form, said infiltration driven primarily by capillarity;
d. subjecting said infiltrated skeleton to temperature conditions
such that said melting point depressant diffuses from said
infiltrated voids into said metal powder; and e. subjecting said
infiltrated skeleton to temperature conditions such that infiltrant
that has infiltrated into said interconnected porosities,
solidifies.
61. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder comprising titanium; b. providing an
infiltrant comprising: i. the same elements as are in the skeleton;
and ii. melting point depressant (MPD) comprising silicon; c.
infiltrating said interconnected porosities with said infiltrant in
liquid form, said infiltration driven primarily by capillarity; d.
subjecting said infiltrated skeleton to temperature conditions such
that said silicon diffuses from said infiltrated voids into said
metal powder; and e. subjecting said infiltrated skeleton to
temperature conditions such that infiltrant that has infiltrated
into said interconnected porosities, solidifies.
62. A method for fabricating a substantially metal part, comprising
the steps of: a. providing a skeleton of interconnected adhered
metal powder having a network of interconnected porosities
throughout, said metal powder comprising titanium; b. providing an
infiltrant comprising: i. the same elements as are in the skeleton;
and ii. melting point depressant (MPD) comprising at least one
material selected from the group consisting of: aluminum, tin,
zirconium, molybdenum, vanadium, copper and chromium; c.
infiltrating said interconnected porosities with said infiltrant in
liquid form, said infiltration driven primarily by capillarity; d.
subjecting said infiltrated skeleton to temperature conditions such
that said melting point depressant diffuses from said infiltrated
voids into said metal powder; and e. subjecting said infiltrated
skeleton to temperature conditions such that infiltrant that has
infiltrated into said interconnected porosities, solidifies.
63. The method of claim 14, further comprising choosing said metal
powder composition, such that relative ratios, of components other
than melting point depressant, are equal to said relative ratios,
of components other than melting point depressant, in said
infiltrant.
Description
The inventions disclosed herein will be understood with regard to
the following description, appended claims and accompanying
drawings, where:
BRIEF DESCRIPTION OF FIGURES
FIG. 1 is a generic equilibrium phase diagram for a mixture of
skeleton material and a single element as a melting point
depressant;
FIG. 2 shows schematically infiltration of an idealized capillary
channel with an infiltrant at liquidus composition and subsequent
diffusion and diffusional solidification to achieve uniform final
bulk composition;
FIG. 3 shows schematically, the percentage of melting point
depressant within the capillary channel and the surrounding
skeleton walls of FIG. 2, at the locations along the capillary
channel designated I and II, at three different times (unprimed, ',
");
FIG. 4 shows schematically dissolution of a pure nickel skeleton
after dipping into an undersaturated (off liquidus) pool of Ni-11
wt % Si infiltrant for 5 minutes at 1200.degree. C.;
FIG. 5 shows schematically erosion at the base of a cylindrical
skeleton, which also progresses several centimeters into the
part;
FIG. 6 shows schematically a skeleton composed of .about.300 micron
powder infiltrated to a height of about 22 centimeters before
freezing choked off the flow of infiltrant;
FIG. 7A is a nickel-silicon equilibrium phase diagram;
FIG. 7B is an enlargement of a portion of the Ni--Si equilibrium
phase diagram of FIG. 7A, which shows schematically an infiltrant
at liquidus composition within a processing window, using the
nickel-silicon binary system as an example;
FIG. 8 is a nickel-phosphorous equilibrium phase diagram;
FIG. 9 is an aluminum-silicon equilibrium phase diagram;
FIG. 10 shows schematically a cross-section of a base powder
particle coated with surface powder 1/50 the size of the base
powder, to increase capillary pressure;
FIG. 11 shows schematically smoothing effect of a moving
solidification front, with the initial surface matching that of
FIG. 10, with the interface shown moving in steps of 1/4 of the
diameter of the surface powder;
FIG. 12 is a digital image that shows schematically a cross-section
showing enhanced surface texture of nickel powder made by
hydrometallurgical processing;
FIG. 13 shows schematically external supply tabs used to feed
infiltrant to several entry points of a part skeleton from an
infiltrant reservoir above the part;
FIG. 14 shows schematically a network of internal feeder channels
built into a part skeleton to facilitate liquid flow to remote
areas without freezing, the channels having variable diameter
(left) or change in direction (right) including both horizontal and
vertical runs;
FIG. 15 shows schematically, in cross-section, surface texture
refinement achieved by adding a paste of fine powder to an external
surface of a skeleton of larger powder, the paste forming a thin
shell outside the skeleton (left) or penetrating and filling the
space between powder near the surface (right);
FIG. 16A shows schematically distortion of a nickel skeleton (first
letter is deformed) which occurred while hanging the skeleton at
1200.degree. C.;
FIG. 16B shows schematically a similar part without distortion that
was resting on a flat crucible bottom;
FIG. 17 is a portion of a titanium-silicon equilibrium phase
diagram;
FIG. 18 is a ternary nickel-silicon-chromium equilibrium phase
diagram at 1250.degree. C.;
FIG. 19 is a ternary nickel-silicon-iron equilibrium phase diagram
at 1200.degree. C., and shows schematically how the relative
proportions of nickel and iron change during diffusional
solidification; and
FIG. 20 is a generic ternary equilibrium phase diagram, and shows
schematically desirable characteristics to achieve uniform final
bulk composition.
DETAILED DISCUSSION
Traditional manufacturing processes using powder metallurgy ("PM")
produce a near net shape part which is only initially 50-70% dense.
These `green` parts then undergo further processing to achieve full
density and the desired mechanical properties either through
lightly sintering and infiltrating with a lower melting temperature
alloy or through a high temperature sintering alone. In the first
method, the part's dimensional change is typically only .about.1%
making it suitable for fairly large (.about.0.5 m on a side) parts,
but the resulting material composition will be a heterogeneous
mixture of the powder material and the lower melting temperature
infiltrant. In the second method, sintering the powder to full
density will result in a homogeneous final material, but a part
starting at 60% density will undergo .about.15% linear shrinkage.
For this reason, full-density sintering is typically only used for
smaller (<5 cm on a side) parts.
In some cases, infiltration can be done extremely rapidly by the
application of external pressure. However, this requires a mold and
typically expensive processing equipment. The inventions disclosed
herein are directed to pressureless infiltration, where the primary
driving force is capillarity and in some cases, gravity.
In many critical applications (structural, aerospace, military), a
material of homogeneous composition (or with homogeneous
properties) is preferable because of certification issues,
corrosion issues, machinability, or temperature limitations that
might be imposed by the lower melting point infiltrant. Further,
because designers of metal components are not accustomed to working
with composites of heterogeneous composition, they experience a
psychological barrier to adoption.
Creation of very large parts with homogeneous composition or
properties via powder metallurgy builds on all of the benefits of
PM processing. This can be done using an infiltration step to
densify the green part without any significant dimensional change,
but in such a way that the final material has a homogeneous
composition or properties to enable significant advantages over
tradition processing. It is also beneficial to ensure the bulk
material composition or properties are consistent throughout the
entire part. Solid freeform fabrication processes, (such as
three-dimensional printing, selective laser sintering, etc.) metal
injection molding, or other PM processes will be enabled to make
homogeneous net shape parts in a wide variety of sizes by methods
described herein. Also disclosed is the potential of matching the
final part composition or properties to existing commercial
material systems.
By three-dimensional printing, it is meant the processes described
generally in U.S. Pat. Nos., 5,204,055, 5,387,380, 5,490,882,
5,775,402, which are incorporated herein by reference.
A general concept, explored more fully below, is to use an
infiltrant composition similar to that of the powder skeleton, but
with the addition of a melting point depressant. The infiltrant
quickly fills the powder skeleton. Then, as the melting point
depressant diffuses into the base powder, in some cases, the liquid
undergoes diffusional solidification and the material eventually
homogenizes. The diffusional solidification may be isothermal, but
need not be. Maintaining the infiltrant at a liquidus composition
for the infiltration temperature typically ensures that the bulk
composition or properties will remain uniform throughout the part,
particularly in the direction of infiltration.
Success of such an infiltration is enhanced by effective means of
maintaining the molten infiltrant at a liquidus composition. It is
also beneficial, in some cases, for the time scale of the
infiltration to be much faster than the time scale of the diffusion
of the melting point depressant and the subsequent solidification
and homogenization. Methods of establishing the liquidus
composition include all of the following or a combination of any of
these: separating the infiltrant melt supply from the skeleton
prior to infiltration, adding excess skeleton material to the melt,
overshooting the infiltration temperature, and agitating the melt.
The relative rates of infiltration and diffusion/solidification
rate are significantly impacted by the choice of materials system.
But other techniques have been developed, and are disclosed herein,
to influence these rates. They include: selection of powder size
(diameter), shape, surface roughness, and size distribution,
feeding liquid from different locations, liquid feeder channels,
smoothing of the part surface with fine powder and affecting the
infiltrant fluid properties.
In some cases, even after the part has reached its equilibrium
condition at the infiltration temperature, some of the infiltrant
in the skeleton will remain liquid after diffusional solidification
has ceased. In some such circumstances, the final microstructure
that results is not homogeneous, but rather is similar to that
typically obtained with a cast part, which is also a useful
result.
Because significant mass transport occurs in parts after
infiltration has occurred, there exists the potential for very
small voids to develop within the part due to differential movement
of species. These voids are generally referred to as Kirkendall
porosity, and may appear to varying degrees based on factors such
as the mechanism of diffusion and the relative size of mobile
species. This porosity is typically very fine scale and may not
affect mechanical properties. Heat treatment, including hot
isostatic pressing, can be used to reduce these voids in the event
they are significant.
Infiltrant with Single Element as Melting Point Depressant
The initial discussion is limited to the important case of an
infiltrant composed of the skeleton material with the addition of a
single element from the Periodic Table to serve as a Melting Point
Depressant (MPD). In this case, it is generally possible to design
the infiltrant composition and the infiltration temperature in such
a manner as to guarantee that the infiltrated body is uniform in
bulk composition, that is, that there is no gradient in bulk
composition along the path of infiltration.
A Melting Point Depressant (MPD) is a material which, when added to
a metal, produces a new alloy which melts at a lower temperature
than the metal itself. The MPD typically is composed of a single
element, however, multiple elements are also possible. The metal
itself may be a single element, however, alloys composed of two or
more elements are most common. Alloys typically have temperature
ranges over which they melt and not just a single melting
temperature. An alloy begins to melt at the solidus temperature and
becomes fully molten at the liquidus temperature. The addition of
an MPD to a metal produces a new alloy with a lower liquidus
temperature (temperature at which the alloy is fully molten) than
the liquidus temperature of the metal itself. (In the case of the
addition of an MPD to a metal composed of a single element, the
liquidus temperature of the alloy is lower than the melting
temperature of the elemental metal.) In a preferred embodiment of
some of the inventions disclosed herein, the MPD will result in an
alloy whose liquidus temperature is below the solidus temperature
of the original metal of a skeleton. In this way, the infiltrant
alloy formed of the metal with MPD added can be fully molten while
the metal is still fully solid and thus infiltration can take place
without the skeleton beginning to melt.
In a binary system there are only two elements present in the
skeleton and infiltrant. In the simplest case, the skeleton is
composed entirely of a single element and the infiltrant is
composed of this element with the addition of a second element as a
melting point depressant. However, the same principles apply if the
skeleton started with some of the MPD in it and the infiltrant
simply has a higher concentration of this MPD.
An equilibrium phase diagram for a generic mixture of a skeleton
material and a melting point depressant (MPD) is shown in FIG. 1.
The infiltration temperature can be chosen anywhere between the
eutectic temperature (.about.1100.degree. C.) and melting
temperature of the skeleton (.about.1440.degree. C.). If the
skeleton is a pure metal, such as nickel, it will have a discrete
melting temperature. If, however, it is an alloy containing two or
more elements, discussed more below, there is a range of
temperatures over which different components of the skeleton begin
to melt. The lower limit of this temperature range is the solidus
temperature of the skeleton, the maximum temperature at which no
liquid is present. Since there is always some variation in
processing temperature, the infiltration temperature should remain
safely below the solidus temperature of the skeleton.
The skeleton will also be prone to sinter and start to sag as it
loses strength near its melting point, discussed below. As long as
the skeleton maintains dimensional stability, the infiltration
temperature can also be selected to influence the diffusivity and
solubility of the MPD in the skeleton material. Generally, the
diffusivity increases and solubility decreases with increasing
processing temperature. These tendencies are relevant to the
challenge of ensuring that the entire skeleton fills with liquid
before solidification chokes off the liquid flow, discussed in more
detail below.
It is helpful to now consider the case where the composition of the
infiltrant liquid at the infiltration temperature lies along the
liquidus of FIG. 1. More specifically, consider the case of
infiltration at temperature T and liquidus composition C. At the
infiltration temperature shown (.about.1300.degree. C.), the
infiltrant would be liquid at any composition between 10% and 50%
MPD. The minimum composition that allows the material to remain
completely liquid is 10% MPD--the liquidus composition at the
designated temperature. Liquid infiltrant at this composition is
considered saturated with the skeleton material, for a binary
system. Any removal of MPD from the infiltrant at this temperature
will result in solidification of some of the infiltrant at the
corresponding solidus composition at point S. Such removal of MPD
will typically take place during the infiltration of the skeleton
by diffusion of MPD into the skeleton. Thus, as the MPD diffuses
into the skeleton, the infiltrant will solidify on the skeleton at
the solidus composition and the remaining liquid will still be at
the liquidus composition--unchanged by the process of infiltration.
Thus the infiltrant flowing through the part will always be at the
liquidus composition and the bulk composition throughout the part
will be ensured to be uniform.
Reference to FIGS. 2 and 3 further illustrates this concept. FIG. 2
shows schematically a saturated melt 110 at a liquidus composition
filling a capillary channel 112. The capillary channel 112 is
formed between two identical sheets 114 of skeleton material with
spacing and void fraction chosen so that the volume fraction of
solid to void space is 60:40. FIG. 2 shows three different moments
in time, with the left most (unprimed) being the earliest and the
right most (double prime ") being the latest. FIG. 3 shows the
expected MPD concentration profile for two locations at the three
moments in time. The vertical axis shows the local composition
(percent of MPD) as a function of the position on the horizontal
axis, with the coordinates c, d, e and f, representing
corresponding locations of the sheets of skeleton material and
capillary channel shown in FIG. 2. Profile I in FIG. 3 represents
the initial condition, when and where the liquid first comes into
contact with the skeleton. This profile is found just below the
meniscus M as the liquid is flowing up the capillary 112 indicated
at I in FIG. 2. At a slightly later time shown in the middle, an
identical condition would be found further along the capillary at
position II'. The composition profile is 10% MPD in the liquid
region and near zero in the solid 114, since the MPD has just begun
to diffuse into the solid 114 at the interface. At the position
marked as I', the liquid has been in contact with the skeleton for
a given time period and diffusion has caused some degree of
solidification of the infiltrant. The composition profile labeled
I' in FIG. 3 corresponds to the cross-section at the position I' in
FIG. 2, with an increased MPD composition in the solid and
resulting motion of the solid/liquid interface inward (from d away
from c and from e away from f). The right most image of FIG. 2
portrays the capillary 112 after the liquid 110 has reached the top
of the capillary channel 114, and the system has had additional
time to equilibrate. Since the composition of all of the liquid
remains constant regardless of whether the liquid is flowing or
not, the solidification behavior and the profile corresponding to
the cross-section at position II" will be identical to I'.
Similarly, point II" at a future moment in time will be identical
to the current profile of I'. After the entire system reaches
equilibrium, the composition profile at I (lower position) and II
(upper position) will be identical and the final bulk composition
throughout the capillary will be uniform.
In contrast, in the case of a liquid infiltrant which has a
concentration of MPD above that of the liquidus composition, (such
as corresponding to the composition for the point indicated at U,
FIG. 1), mass transport of both the skeleton material and the MPD
has the potential to change the infiltrant composition. If this
happens while the liquid is still flowing into the skeleton, the
last areas of the skeleton to be reached by the liquid will have a
different final composition of MPD than the first areas reached.
Such a variation in bulk composition throughout the part could not
be rectified by a homogenizing heat treatment in a reasonable
time.
To visualize this concept, consider another similar idealized
capillary channel made by two identical sheets of skeleton material
with spacing chosen so that the volume fraction of solid to void
space is 60:40. Filling the void space within the control volume
with an infiltrant composed of 15% MPD would result in an average
bulk composition of 6% MPD. For the phase diagram shown in FIG. 1,
the infiltrant would be unsaturated (off the liquidus) at the
infiltration temperature. The infiltrant would exchange mass with
the skeleton material upon contact at its entry point, until
reaching its equilibrium liquidus composition of 10% MPD. For the
case of a capillary being filled from the bottom, this could occur
while the liquid is flowing through the capillary such that the
bulk composition at the top would be only 4% MPD, due to MPD
depletion. At the bottom (the entry point), the bulk composition
would be greater than 6% MPD because of the loss of skeleton
material and subsequent replacement with infiltrant having a
composition of 15% MPD. Such a variation of bulk composition would
result in undesirable variation of properties throughout an
infiltrated part. In simple terms, this is the result of lower
sections of a part skeleton being dissolved into the liquid and
then carried by the liquid to other regions of a part. (If
infiltrant enters the part from its top rather than its bottom,
such as by simply placing a slug of infiltrant supply material on
top of a skeleton and heating it, then the top regions would have a
higher contribution based on the MPD. Basically, the region
adjacent the infiltrant supply will have increased contribution to
composition from MPD.) The case of ternary and higher compositions
is similar, with an important added consideration. It is still true
that the infiltrant should be designed and controlled to be at a
liquidus composition for the chosen temperature of infiltration
(see subsequent discussion on erosion). It is further true that the
removal (by diffusion) of any of the MPD will result in
solidification of material at a solidus composition. However, in
the case of a ternary system, for example, at a single temperature
there are a range of possible liquidus compositions and a range of
possible solidus compositions (as opposed to just a single liquidus
and a single solidus composition in the binary case).
FIG. 18 shows a ternary phase diagram for the system consisting of
nickel, chromium and silicon at a temperature of 1250.degree. C.
(This and all subsequent ternary phase diagrams were generated
using Thermo-calc, a Computational Thermodynamics program used to
perform calculations of thermodynamic properties of multi-component
systems based on the Kaufman binary thermodynamic database.) Line
780 is the liquidus (at this temperature) and any composition
falling along this line is a liquidus composition. Line 782 is the
solidus and any composition falling along this line is a solidus
composition. Further, tie lines 784 connect specific liquidus and
solidus compositions. The liquidus and solidus compositions at the
end of each tie line can co-exist in two-phase equilibrium.
To ensure that the infiltrated part does not develop a composition
gradient along the path of infiltration due to diffusional
solidification during infiltration, the liquid infiltrant must
solidify with no change in the relative contributions to the
infiltrant composition of the non-MPD elements. To illustrate,
consider the general case where an infiltrated part would develop a
gradient in bulk composition due to diffusional
solidification--shown schematically in FIG. 19 using the ternary
phase diagram for the system consisting of nickel, iron, and
silicon at a temperature of 1200.degree. C. In this case, line 890,
which passes through the corner of the diagram corresponding to
pure Si, corresponds to compositions that have a constant ratio of
Ni and Fe (approximately 72:28). There are other lines of constant
ratio, which represent different ratios. A liquidus composition
marked 820 is on the line. The solidus composition marked 822 that
is connected to the liquidus by a tie line, is not on the line.
Thus, the solidus composition has a different ratio of Fe to Ni
than does the liquidus composition. In such a case, if the
infiltrant begins at a liquidus composition such as 820,
diffusional solidification of the infiltrant will result in the
remaining liquid becoming relatively richer in Ni (and poorer in
Fe). This Ni enriched infiltrant will travel up the skeleton,
resulting in the further reaches of the infiltrated part having a
higher composition of Ni than the first areas to infiltrate. This
is somewhat undesirable.
The desirable circumstance is best illustrated by reference to FIG.
20, a ternary phase diagram identical to FIG. 18 with the elements
labeled A, B and MPD to represent a generic system. In this case,
line 990, which passes through the corner of the diagram
corresponding to pure MPD, corresponds to compositions that have a
constant ratio of A and B, the non MPD components of the
infiltrant. The desirable case then is an infiltrant liquidus
composition whose tie line lies along this line, as shown. In this
case, the liquidus and solidus that are in equilibrium with each
other have the same relative concentration of A and B and
diffusional solidification will not result in a change in the
relative composition of A and B. This will guarantee that there is
no variation of composition along the path of infiltration. Not all
material systems and infiltration conditions will allow for this
condition. Rather, the material system, liquidus composition
(infiltrant composition) and infiltration temperature must be
chosen by these criteria.
In the most desirable circumstance, the tie line lies along a line
of constant relative proportions of A and B as above, and the
composition of the skeleton also lies on this same line of constant
relative proportions of A and B. In the case of FIG. 20, the
skeleton material composition could be chosen at point 992. Thus,
as the infiltrant undergoes diffusional solidification, the
solidified infiltrant has the same relative proportions of A and B
as the skeleton and this will be true along the entire path of the
infiltrant. Thus, there will be no need to wait for diffusion of
either or both species A and/or B between the skeleton and
solidified infiltrant in order to attain uniform composition
between them.
In the case of ternary and higher alloys, not all materials systems
will allow for the selection of infiltrant alloy such that the tie
line is along a line of constant relative proportions of A and B.
Further, the added desirable feature of having the skeleton
composition lie on this same line of constant relative proportions
is more restrictive.
Thus, an aspect of one of the current inventions is to select and
design materials systems according to the criteria described. This
includes the selection of the elements in the skeleton, the
selection of the MPD, the selection of the relative amounts of the
non-MPD elements in both the skeleton and in the infiltrant (if
different), and the selection of the infiltration temperature. The
most preferred case is that the tie line lies along a line of
constant relative composition of non-MPD elements and that the
skeleton composition lies on this same line. However, having the
tie line lie on this line with the skeleton composition not lying
on this line is sufficient to guarantee uniform composition along
the infiltration path. Uniform composition between infiltrant and
skeleton might then be attained by diffusional homogenization. Note
that a change in the infiltration temperature will change the
orientation of the tie lines and so, the selection of infiltration
temperature must also be based on this consideration.
The principles explained herein in the context of the ternary phase
diagram also apply to systems with four or more alloying elements.
In particular, a system is to be chosen such that, during
diffusional solidification, the relative ratio of the non-MPD
elements remains substantially constant and, preferably that this
ratio is substantially the same in the skeleton as in the
infiltrant.
Methods of Ensuring a Liquidus Composition (Saturation)
If the infiltrant composition is known exactly, the process
temperature can be selected to exactly match the liquidus
temperature for that composition, but this requires very accurate
process control. A more robust method for ensuring that the liquid
lies at an appropriate liquidus composition, is to put the liquid
in contact with sacrificial excess solid skeleton material and
allow it to reach an equilibrium liquidus composition corresponding
to the actual processing temperature. Having a high interfacial
surface area between the liquid and solid can help promote mass
transport and speed the process of equilibration. For this reason,
it is beneficial if the excess solid material is supplied in powder
form, which has large surface area, but this alone may not suffice
to guarantee reaching the liquidus composition in reasonable times.
In the case of a binary system, as explained above, the liquidus
composition is also referred to as a composition saturated with
skeleton material.
If raising the MPD concentration lowers the liquid density (which
is typically the case), the liquid will stratify with the higher
density liquid of low MPD concentration in contact with the solid
at the crucible bottom. The liquid of higher MPD concentration will
remain at the surface with no mixing by natural convection.
Stirring the melt or using some other means of agitation to force
convection promotes mixing. A ceramic propeller has been used to
stir the infiltrant supply by running a shaft through the furnace
roof with a Teflon seal a small motor to power the propeller. Other
possible mechanical stirring methods that may be used include
tipping the crucible back and forth, flowing the liquid through a
sacrificial porous network of skeleton material, shaking, vibrating
or sonicating the melt, or bubbling gas through the melt. Placing
the liquid in an inductive AC electromagnetic field can also
generate substantial mixing by inducing currents in the molten
metal. Heating the infiltrant supply by induction is one means for
preparing a well-mixed liquid at the equilibrium liquidus
temperature. (As used herein, "infiltrant" typically means liquid
material that actually infiltrates a skeleton. "Infiltrant supply"
means the source material that will melt to become the infiltrant.
The infiltrant source material becomes liquid infiltrant plus
residual solid at the infiltration temperature; liquid infiltrant
is what is available to enter the skeleton.)
Another method to help ensure the liquid is at its liquidus
composition is overshooting the infiltration temperature to
dissolve excess solid skeleton material that has deliberately been
added to the melt. Once the excess material is dissolved, the
temperature is slowly ramped back down to the infiltration
temperature while agitating the melt. This promotes
re-solidification of material, with the remaining liquid at the
desired liquidus composition. As long as the liquid is in contact
with some solid, it is unlikely that any under-cooling would
occur.
The amount of excess skeleton material added to the melt must be
sufficient to saturate the melt, but not so much that the melt
solidifies. The proper amount is a function of the skeleton
material's solubility (maximum capacity to absorb MPD). For
example, with a nickel-silicon infiltrant, sacrificial excess
nickel powder is added to the crucible of infiltrant. The
appropriate amount is determined by considering the extreme cases
for a range of processing temperatures. FIG. 7A shows an
equilibrium phase diagram for nickel and silicon, and FIG. 7B shows
an enlargement of a portion of FIG. 7A. FIG. 7B illustrates how
this would be done for a desired infiltration temperature of
1180.degree. C. and maximum temperature variation (due to
uncertainties) of plus or minus 20.degree. C. Above and to the
right of the liquidus, all compositions are liquid. Below and to
the left of the solidus, all compositions are solid. Between, there
are compositions that have two coexisting phases, liquid and solid.
The bulk composition of infiltrant supply is chosen from the
intersection of the maximum anticipated temperature (in this case
1200.degree. C.) with the liquidus line, marked as A on FIG. 7B (in
this case, 10% Si and 90% Ni). This ensures that some solid will be
present at any temperature below this expected maximum temperature
and all of the liquid present will be saturated with nickel and be
at the liquidus composition for that temperature. If the
temperature is at the lower limit, the total amount of the material
provided as the infiltrant supply will be partially liquid and
partially solid, in a two-phase field between the liquidus and the
solidus. The ratio of liquid to solid will be given by the lever
rule. For this example, at 10% Si and 1160.degree. C., it would be
approximately 30% solid. This will determine the total quantity of
infiltrant supply needed, since only 70% of the infiltrant supply
is guaranteed to be liquid infiltrant available for filling the
part in this example.
For the case of ternary or higher alloys, there can be a range of
liquidus compositions at a given temperature. The infiltrant supply
composition can be selected such that it contains the elements of
the skeleton material in a relative ratio similar to their
elemental composition in the skeleton material alone. Once again,
the bulk infiltrant supply composition can be selected to lie in
the two-phase region between the solid and liquid at the
infiltration temperature, such that any liquid present will be at a
liquidus composition. As described previously in FIG. 19, the
solidus and liquidus compositions corresponding to a given tie line
may have different relative proportions of the elements of the
skeleton material. Solidification due to diffusion of MPD may
result in depletion or enrichment of other species in the liquid,
but this would only result in the liquid moving to a different
liquidus composition. The new liquidus composition would still not
allow dissolution of any skeleton material. Any enrichment or
depletion of the solidifying composition would come from the
changing liquid composition rather than the existing skeleton
material.
Erosion
As mentioned above, if the liquid infiltrant has a composition that
is not saturated in the skeleton material, in other words, where
the concentration of the MPD is greater than in the equilibrium
liquidus composition for a given temperature, the liquid infiltrant
will have the capacity to absorb additional material from the
skeleton and partially dissolve the skeleton. This can happen very
quickly, because of high diffusivity in liquids and can be a
significant problem, especially when a large melt pool is used.
FIG. 4 shows a pure nickel skeleton, originally a cylinder, with
its bottom section dissolved. It was dipped into a pool of molten
Ni-11 wt % Si for 5 minutes at 1200.degree. C. Since the
equilibrium liquidus composition has significantly less than 11 wt
% Si at that temperature, the pool of liquid absorbs the solid
nickel as it contacts the skeleton.
To a lesser degree, a liquid infiltrant not already at its
equilibrium liquidus composition has a tendency to leave an erosion
path as it enters the skeleton. This occurs to some extent in most
powder metal infiltrations, but usually is limited to the initial 1
cm at the base of a part. In such cases, the part to be infiltrated
can be placed on a sacrificial stilt. In the nickel-silicon system
with an unsaturated infiltrant containing more than the liquidus
silicon concentration, the erosion tends to propagate for several
centimeters into the part and resembles a riverbed (one example is
shown in FIG. 5). This part is approximately 10 cm. long. It was
infiltrated from the end near to the zero of the scale, the bottom
of FIG. 5, as shown. Studying the erosion pattern on several
different shaped parts suggests that erosion occurs in the areas of
highest liquid flow. Once erosion begins, a larger channel is
created, which has less viscous drag and allows even more liquid to
flow through the newly formed channel. An instability such as this
explains why the erosion progresses so far into the part (almost 4
cm.). Through metallographic study of cross sections, the eroded
areas are found to be high in silicon content. This is not
surprising, since those compositions would be liquid at the
infiltration temperature. The areas of erosion are not limited to
the surface. Voids have been found within the interior of a part in
a region of high silicon content.
Using a liquid infiltrant that is at its equilibrium liquidus
composition before it contacts the skeleton removes the driving
force for diffusion and has proved to be a good method of
preventing erosion. Since temperature affects both the diffusion
rate and the infiltrant liquidus composition, temperature variation
with time or a temperature gradient set up within the part, could
be used as a further method of erosion prevention.
Infiltrant with Multiple Elements used to Depress Melting Point
Two or more elements can be used as a melting point depressant. In
that case, the mass transport can become more complicated and
having a liquidus composition no longer provides a guarantee of
uniform bulk composition throughout the part. The diffusivity and
solubility in the skeleton material of the various MPD elements
will determine their mass transport during the infiltration. If the
elements behave similarly to each other, the bulk composition would
likely become uniform for the same reasons discussed in the
previous section. However, significant diffusion of one element
without the other, while the liquid is still flowing, would likely
result in variation of final bulk composition. This is the more
typical situation. For example, if a second element has
significantly lower solubility in the skeleton material, less of
that element will be absorbed in the solidifying material and the
remaining liquid will be enriched in that second element. This
enriched liquid would be carried to other regions of the part,
while a fresh supply of infiltrant at the original composition
replaces it.
In these cases of multiple elements having different mass transport
properties, uniform bulk composition can be achieved by filling the
part with liquid in a much shorter time scale than the time scale
of diffusion and solidification. Factors that influence the rate of
filling and of diffusion are discussed below.
Relative Rates of Infiltration and Diffusion
Due to the diffusion of MPD into the skeleton and corresponding
infiltrant solidification, in many cases the liquid has only a
limited time to fill the part skeleton before the flow is choked
off by solidification. In addition, fast infiltration relative to
diffusion may be necessary to ensure uniform bulk composition as
mentioned above for cases with multiple elements diffusing. The
rate of infiltration is determined by various factors, including:
the surface tension, wetting angle, viscosity, and density of the
liquid infiltrant as well as the geometry of the skeleton,
determined by powder size and shape, size distribution, packing
density and part geometry. Specific relations are provided below in
the discussion of each topic. FIG. 6 shows a Ni--Si skeleton which
has been infiltrated to a height of 22 cm by controlling the
relative rates of infiltration and diffusion such that diffusion
had not proceeded to the point of choke off before the infiltrant
reached 22 cm.
Some typical infiltration rates have been measured of the Ni-10 wt
% Si infiltrant, filling a skeleton of 50-150 micron nickel powder.
This was done by hanging the skeleton from a wire through the roof
of the furnace and measuring the force on the wire. By compensating
for the surface tension and buoyancy forces, it was possible to
relate the force to the increasing mass of the part due to the
addition of liquid. The liquid filled an 8 cm tall skeleton in
approximately one minute. Other liquid metals have similar
viscosity and surface tension so this rate should not change
drastically with material system.
The diffusion rate controls diffusional solidification, a special
case of which is isothermal solidification of the infiltrant and
the eventual homogenization of the skeleton. Since, in a typical
case, the liquid fills a small skeleton in approximately one
minute, diffusional solidification would ideally take place over a
much longer time period, e.g., an hour or two. The diffusion rate
will be controlled primarily by the material system chosen.
Selection of a material system is critical to controlling the time
scale of the isothermal solidification. In particular, the
diffusivity of the melting point depressant in the solid skeleton
will have the greatest effect on the freezing. The basic components
of the skeleton are usually dictated by other requirements. A part
is typically specified as steel, or aluminum, etc. and thus, that
metal or an alloy thereof will be the basic component of the
skeleton. Part geometry is also typically not a variable the
process designer can change. Using a slower diffusing melting point
depressant can drastically increase the amount of time the skeleton
has to fill with infiltrant before freezing begins to occur. Si, B
and P can all be used as a melting point depressant in Ni. Of
these, Si diffuses much more slowly than do B and P. Diffusivity
also has a strong dependence on temperature, since it is an
activated process that follows Arhennius dependence. Controlling
infiltration temperature allows for some control of the diffusivity
for a given material system. Reduced temperature decreases
diffusivity and should allow more time for the liquid to fill the
skeleton before freezing.
Coating the powder skeleton (or just the raw powder) with a finite
time diffusion barrier slows the freezing by keeping the melting
point depressant from leaving the infiltrant until the liquid has
filled the part. Such a diffusion barrier can be another metal that
has a lower diffusivity of MPD. The thickness of the barrier can be
selected so that it only lasts for the duration of the
infiltration. As the coating material begins to break down, it
allows the MPD to diffuse through, allowing isothermal
solidification and eventual homogenization. Ideally, the coating
material itself is also relatively homogeneous throughout the
part.
Distinction Between Low and High Solubility Systems
Returning to the case of a single element as melting point
depressant, the solubility of the melting point depressant in the
skeleton material also influences the diffusional solidification
behavior. The solidus line on the phase diagram describes the
solubility of the MPD in the solid, and the location of the final
part bulk composition relative to this line will determine whether
the part will completely solidify at that temperature or if there
will always be liquid present at the infiltration temperature. The
two most important factors in determining the bulk composition of
the final part are: the packing density of the powder skeleton
(which determines the liquid void fraction); and the infiltration
temperature (which determines the infiltrant composition for a
given material system assuming the infiltrant is at the liquidus
composition).
In the first case of relatively high solubility, such as Ni--Si,
characterized by the equilibrium phase diagram shown in FIG. 7A,
solidification will proceed to completion and choke off the flow of
liquid. It is therefore necessary to ensure that the liquid fills
the entire part before the infiltrant solidifies. Several
mechanisms for increasing the rate of infiltration relative to the
rate of solidification are presented below.
As an example, a pure nickel skeleton infiltrated with an alloy
containing silicon as a melting point depressant would likely fall
into this category. The equilibrium phase diagram for Ni--Si is
shown in FIG. 7A. Assuming a skeleton with packing fraction of 60%
and an infiltration temperature of 1180.degree. C., a bulk
composition of .about.4% would result from the liquidus composition
of .about.10% times the liquid volume fraction 0.4. Since this is
significantly less than the solidus composition of .about.7%, such
a part would completely solidify diffusionally.
In a second case, of low solubility, when the bulk composition of
the part lies in a two-phase equilibrium field, liquid will remain
present in the infiltrated skeleton until the part is cooled below
the infiltration temperature. It is possible that the liquid volume
fraction may remain high enough to allow continuous flow through
interconnected pores, such that any diffusional solidification that
occurs would not prevent the part from being completely
infiltrated. For example, consider a pure nickel skeleton
infiltrated with a nickel alloy containing phosphorous as a melting
point depressant. The Ni--P equilibrium phase diagram is shown in
FIG. 8, with the liquidus line indicated at 270. The solidus line
272 is not discernible on this diagram, because it is so close to
0% P line, between 1455.degree. C. and 870.degree. C. A lower
infiltration temperature of 1000.degree. C. could be used and the
liquidus composition of 7% P would result in a bulk composition of
2.8% P. Since the solubility of P in Ni is only 0.17%, only a small
amount of P would diffuse into the skeleton at the infiltration
temperatures and there would be very little solidification and
restriction to the liquid flow.
Another material system with low solubility is the binary alloy of
aluminum and silicon, with the equilibrium phase diagram shown in
FIG. 9. The liquidus 280 and the solidus 282, along with the line
at 577.+-.1.degree. C. bound a two-phase region. At an infiltration
temperature of 600.degree. C., there is approximately 2% solubility
of silicon in aluminum. If liquid infiltrant at a composition of
10% Si filled the void space of a 60% dense pure aluminum skeleton,
the bulk part composition would be 4% Si and would lie in a
two-phase field of .about.25% liquid as long as the skeleton
remained at the infiltration temperature. In this case, the
skeleton would absorb some Si, but diffusional solidification would
occur only until the part was 75% solid. The liquid flow would not
be completely choked off by the solidification, so the rate of
diffusion and solidification need not be slow compared to the
infiltration rate.
The final microstructure resulting in these cases of low solubility
of the MPD in the skeleton material will not be a single-phase
solid solution of the MPD in the skeleton material. The original
interconnecting porosity space that was filled with liquid will
have a two-phase microstructure. The aluminum-silicon system's
result would resemble that of a cast microstructure. The powder
particles that have absorbed Si would substitute for the primary
dendrites that solidify first in a casting. The remaining
infiltrant would have a eutectic microstructure similar to the
regions of a casting between the dendrites that are the last to
freeze. Since this type of microstructure is sometimes desirable
and widely accepted in industry, the fact that the material
composition is not uniform throughout is not a drawback. Two-phase
strengthening is common for commercial net-shape casting alloys and
can also be achieved in cases of infiltrated systems with low
solubility.
By contrast, high solubility cases are more typical of commercial
wrought alloys, relying on solid solution strengthening or
precipitation hardening. Either the high or low solubility case
will result in more uniform properties than traditional
heterogeneous infiltration and will eliminate the disadvantages of
poor machinability, poor corrosion resistance, temperature
limitations, and difficulty in material certification.
It should be noted that even when diffusional solidification takes
place, it is not necessary to wait for it to complete before
lowering the processing temperature. Once the infiltrant has
completely filled the skeleton, the skeleton can be cooled to
another temperature, subsequent solidification and homogenization
can continue to take place by diffusion. This could be useful
because the solubility of the MPD in the skeleton typically changes
with temperature.
Gating
If the infiltrant begins to wick in to the part as soon as a small
portion of the infiltrant supply is molten, two problems result.
First, as soon as infiltration begins, diffusion and homogenization
also begin and the pores of the skeleton may become occluded by
material that has undergone diffusional solidification. Thus, a
small amount of molten infiltrant may cause the pores to clog
before the general mass of infiltrant supply is available to enter
the body. Second, if the infiltrant supply, during its preparation,
has solidified into multiple phases (which will generally be the
case), these phases will melt sequentially as the infiltrant supply
heats up. Thus the first liquid available to enter the skeleton
will not have the average composition of the infiltrant supply.
These problems can be avoided by first creating a melt of
infiltrant and allowing it to equilibrate thermally and chemically
before putting it in contact with the skeleton to be
infiltrated.
Further, it is advantageous to preheat the skeleton. If the
skeleton is not preheated, the infiltrant will heat up the exterior
of the skeleton where they contact each other and the infiltrant
will begin to penetrate. The rate of penetration will be limited by
the heating of the skeleton rather than just fluid mechanics, since
the liquid would be unable to flow into colder areas of the part.
In these initially penetrated regions, diffusion will begin upon
contact and the pores may become choked off and prevent subsequent
flow. Since the infiltration of the skeleton as a whole is limited
by the need to heat up the interior of the skeleton, this problem
can be avoided by preheating the entire skeleton.
Several gating methods have been used to initially separate the
melt from the skeleton, then control the introduction of the
liquid. By "gating," it is meant mechanically separating the
skeleton and the liquid infiltrant supply, and then bringing them
together. The motion of a linear or rotary feedthrough from outside
the furnace can be translated to open a `gate` and introduce the
liquid to the skeleton.
One gating method is to suspend the skeleton before infiltration
and dip it into a pool of the molten infiltrant. Either the
skeleton can be lowered, or the pool can be raised, or both, to
bring the skeleton and the pool together.
If the skeleton is too delicate to hang under its own weight, then
a mechanism should be used to allow a gated infiltration with the
part resting in a crucible. It can be difficult to create a
hermetic fluid seal that will hold at the infiltration temperature,
but using a crucible material that is not wet by the infiltrant
makes a seal possible. Two such mechanisms have been used
successfully. The first is a vertical alumina plate used to
separate a rectangular crucible into two halves. The shape of the
plate must match the cross-sectional profile of the crucible, so a
bisque-fired alumina plate was cut and filed to maintain less than
1 mm gap when fitted to the crucible. This gap was sufficient to
hold a 2 cm deep pool; a deeper pool would require closer
tolerances or filling of any gaps with a coarse alumina powder. A
more elegant solution is to use an alumina tube with a cleanly cut
end to sit vertically, with the end flush with the bottom of the
crucible. The infiltrant supply is placed inside the tube and the
melt is contained until the tube is lifted from above.
Several other methods can be used for gating the infiltration. One
method involves a custom crucible that has a hole at the bottom.
This hole is plugged with a ceramic rod to prevent infiltrant flow
until the rod is removed. The infiltrant flows through the hole
into another vessel, below, that holds the skeleton. Another method
is to tip a container of infiltrant supply, allowing the liquid to
flow out of the container. Further, the vessel used to contain the
infiltrant supply can be flexible. A woven cloth of alumina fibers
has been used to contain liquid metal. Such a cloth bag can be used
to contain the melt and then opened up to allow the liquid to flow
out.
The actuation of any type of gate requires a linear or rotary
motion actuator passing through the gas-tight shell of the furnace.
In the case of nickel parts fired in a forming-gas atmosphere, the
feedthrough can be a rod sliding through a slightly oversized hole
in the shell. If the internal pressure in the furnace is maintained
to several inches of water, the leak will not allow air into the
furnace to contaminate the atmosphere. In applications where
atmosphere purity is more critical, several linear and rotary
motion feedthroughs designed for high vacuum applications are
available commercially.
Powder Size and Size Distribution
The choice of powder size, defined for spherical powder by the
diameter, has a substantial effect on the depth of penetration of
infiltrant into the skeleton. For simplicity of discussion, the
case of particles which are spherical and which are substantially
mono-modal will be considered. This means that a given skeleton is
made of spherical particles that are all approximately the same
size. Initially, it will be assumed that the particles have smooth
surface texture. (Particles with non-smooth surface texture are
discussed below). Four physical phenomena are influenced by the
particle size:
1. The capillary pressure developed by the infiltrant increases as
particle size decreases. The capillary pressure is the pressure
developed across the interface between the ambient gas and the
liquid infiltrant due to the curvature of the surface of liquid
infiltrant between the skeleton particles. It is the capillary
pressure that causes the infiltrant to be wicked into the skeleton.
Thus, other things being equal, the higher the capillary pressure,
the faster the infiltrant will wick into the infiltrant. If the
infiltrant wicks in quickly, it can penetrate farther before any
choking off of the pores due to diffusional solidification. An
expression for the capillary pressure may be developed by applying
the Laplace equation to the meniscus between the particles.
Alternatively, the capillary pressure .DELTA.p may be expressed as
a function of the surface area per unit volume of a powder bed as
follows [G. Scherer, "Theory of Drying," J. Am. Ceram. Soc., 73,
pp. 3-14 (1990).]: ##EQU1##
where .gamma..sub.LV is the liquid/vapor interfacial energy,
.theta. is the contact angle of the liquid with the solid, S.sub.p
is the surface area of the pore space and V.sub.p is the volume of
the pore space. For mono-modal spheres it can be shown that
##EQU2##
where .epsilon. is the void fraction and D is the powder
diameter.
2. The maximum height to which infiltrant can rise in the skeleton,
(in the absence of diffusional solidification and choking off of
the pores) increases as particle size decreases. This effect is due
to the increase in capillary pressure with decreasing particle
size. As the infiltrant rises up the skeleton, the capillary
pressure must be sufficient to overcome the pressure due to the
static head of the liquid metal in the skeleton. This static head
is related to the density of the liquid .rho., acceleration of
gravity g, and height h above the free liquid surface as follows
[James A. Fay. Introduction to Fluid Mechanics. MIT Press:
Cambridge, Mass. 1994.]:
The maximum possible height of this liquid is attained when the
gravitational head (Eq. 2) is equal to the capillary pressure (Eq.
1).
3. Increasing the size of the particles leads to larger pore spaces
between them and a reduction of the effect of viscous drag on the
flowing infiltrant. Darcy's Law describes how the pressure gradient
in a porous medium is directly proportional to the volume-averaged
velocity of the fluid: ##EQU3##
where .mu. is the fluid viscosity, and K is the permeability of the
medium [Fay].
For the case of mono-modal smooth spherical powder, the
permeability of a powder bed can be predicted by the Carman-Kozeny
relation [Phillip C. Carman. Flow of gases through porous media.
Butterworths:London. 1956.]: ##EQU4##
where .epsilon. is the void pore fraction of the powder bed and S
is the specific surface area, which is equal to 6/D for mono-modal
spheres.
Thus, other things being equal, it is believed that a powderbed
with a higher permeability will allow the liquid infiltrant to
penetrate faster and therefore penetrate farther before the pores
are choked off by diffusional solidification.
4. Increasing the size of the powder reduces the surface area of
the powder per unit volume of the skeleton. The diffusion of the
melting point depressant occurs through the surface and therefore
reducing the surface area in turn slows down the diffusional
solidification and allows for a greater infiltrant penetration
distance before the pores are choked off. Similarly, larger powder
requires the MPD to diffuse over a longer distance to reach the
interior volume of each particle. Thus, other things being equal,
it is believed that increasing the size of the powder results in a
longer time available for infiltration before diffusional
solidification chokes off the pores and therefore, greater
penetration.
It is important in many cases to attain greater penetration of the
infiltrant before choke-off occurs. Thus, the powder size is a very
important variable. If the skeleton is made from very fine powder
(for example metal powder of 20 microns and smaller, down to, for
instance, even 1-3 microns), the capillary pressure will be high
and the maximum height to which infiltrant can rise will be high.
However, the viscous drag of the penetrating infiltrant and the
surface area available for diffusion leading to isothermal
solidification will also be high. Because decreasing the powder
contributes toward the greater penetration distance in two ways and
also detracts from greater penetration distance in two ways, the
details of the relationships must be examined to gain guidance
about the choice of particle size that will maximize penetration
distance.
To understand the relationships, a relatively simple system is
considered first. In this simple system no diffusion, and
therefore, no diffusional solidification, takes place. Further, the
melt is penetrating horizontally relative to a vertical
gravitational field. In such a case, only two of the four factors
above are operative--the change in capillary pressure with particle
size and the change in viscous drag with particle size. While these
factors act in opposite directions, the viscous drag is sensitive
to particle size squared, while the capillary pressure is only
directly proportional to particle size, as can be seen from
equations 1, 3 and 4. In other words, as particle size increases,
the viscous drag drops off faster than the capillary pressure. The
result is that the penetrating liquid moves faster through the
skeleton as the particle size increases.
The next case to consider is one where diffusion and diffusional
solidification take place also. The only one of the four effects
listed above which is not at play is the need of the infiltrant to
rise against gravity. An increase in powder size will even more
strongly favor penetration, because increased powder size reduces
the rate of diffusion of the melting point depressant into the
powder.
Only when one considers an infiltration that is proceeding
vertically (against gravity), does one see an effect that limits
the effectiveness of increased particle size in attainment of
greater penetration distance. As the particle size is increased,
the maximum height to which infiltrant can rise, decreases. Thus,
as particle size is increased, the height attained by the
infiltrant will increase only up to this limit imposed by
capillarity and gravity. In fact, as this limiting height is
approached, the infiltration will proceed ever more slowly, as
there will be little pressure remaining to drive the flow.
Diffusion and diffusional solidification will have more time to act
and thus, it will be difficult to ever attain the full value of
this limiting height.
A discussion of a method of designing a process to manufacture a
part follows. The designer is typically faced with infiltrating a
body of a height specified by the design project. In such a case,
the designer would first choose a relatively small particle size
(to attain the best surface finish possible) and increase the size
of the particles as needed to gain infiltration throughout the body
and up to the top (if proceeding vertically against gravity) of the
designed part. However, if the body is too tall for infiltration it
may not be possible to pick a particle size large enough, because
the limitation imposed by the gravitational head may be reached
before reaching the top of the part. The combination of equation 1
and 2 predicts the maximum capillary rise height. For example, for
liquid metal, with a surface tension of .about.1 N/m, density of 8
g/cc, and a 60% dense skeleton of 250 micron diameter powder, the
rise height would be: ##EQU5##
The discussion above has been in the context of substantially
mono-modal powders. In a bimodal powder, where fine powder is added
and used to fill the interstices between the larger powder, the
fine powder increases the capillary pressure, but it also very
substantially increases the viscous drag and results in a decrease
in the infiltration speed of the molten infiltrant.
Surface Area
A further method to attain greater penetration distance of the
molten infiltrant before diffusional solidification is to increase
the surface area of the powder, but without changing its basic
size. FIG. 10 shows schematically a powder particle 330 which has a
texture on the surface resulting in increased surface area. By such
means, it is possible to increase the surface area of a powder
particle by a factor of two or more. The capillary pressure is
related to the surface area per unit volume. Thus such texturing
will increase the capillary pressure proportional to the increase
in surface area because the volume remains approximately the same.
Further, such texturing has only minimal effect on the size and
shape of the pore spaces between the particles and thus has minimal
effect on the viscous drag of the infiltrant through the skeleton
(although the roughness does very slightly increase the drag).
Following the reasoning above, the penetration of a non-diffusing
infiltrant is faster in a skeleton made with powder with non-smooth
surface texture, because the capillary pressure increases much
faster than the small increase in viscous drag. The increase in
surface area will, however, lead to an increase in the rate of
diffusion in the case of a melt with a diffusing species. However,
this increase in diffusion will apply only at the initial contact
between the melt and the powder, because the initial solidification
will tend to smooth out the powder particle, as shown in FIG. 11.
The solid/liquid interface is moving in the direction of the arrow
marked A. The initial surface 332 has relatively sharp indentations
and greater surface area, as compared to subsequently formed
surfaces 334, 336. Thus, the net effect of surface texture on the
penetration of a melt with a diffusing species is beneficial--that
is, greater penetration distance before diffusional
solidification.
FIG. 12 is a digital image that shows a cross section through
nickel powder particles made by hydrometallurgical processing. This
process results in some degree of surface texturing of the type
desired. For the particle illustrated, the increase in surface area
over a spherical particle is only about 25 percent. Changes in the
deposition parameters may result in a more accentuated surface
area. A method to achieve a surface texture similar to that shown
in FIG. 10 is to coat large powder particles with a single layer of
much finer powder (50:1 powder diameter ratio shown in the figure)
and to sinter that finer powder particles into place. In general,
the coated powder is between 10 and 1000 times the size of the
coating powder, and preferably between 20 and 200 times the size.
For example, 200 micron nickel powder is coated with 2 micron
nickel powder. In principle, an increase of a factor of five in
surface area is possible using such a technique. Alternatively,
etching techniques can be used to create surface textures. One such
technique is vapor-phase etching. This would tend to create
grooving along the grain boundaries and other crystallographic
defects in the powder.
Fluid Supply Tabs
To fill skeletons with dimensions larger than the penetration
distance limit due to freezing, other techniques are required.
Variation of the entry point of the liquid infiltrant can be used
to alleviate some of these problems. FIG. 13 shows a skeleton 370
and an infiltrant reservoir 372. Infiltrant can be supplied to
multiple areas 374, 376, 378, 380, 382 of the part 370 rather than
just the bottom surface. External fluid supply tabs 384, 386, 388,
390, 392 can bring liquid infiltrant to any area of the skeleton's
surface. This reduces the limitation on a maximum dimension to a
less stringent limitation of maximum part thickness. FIG. 13 shows
the tabs supplying infiltrant with the aid of gravity, in which
case they could be hollow tubes allowing the infiltrant to easily
flow through them. However, they can also be arranged to provide
fluid from a melt pool underneath the part. In this case, they
would need to have a porosity suitable to draw infiltrant up to the
required height by capillarity. The tabs could be inert relative to
the infiltrant to prevent any change in composition or undesirable
closing off of pores. The porosity of the tabs would be relatively
coarse as compared to the skeleton to permit liquid infiltrant to
travel through quickly.
Feeder Channels
Also, as shown in FIG. 14, solid freeform fabrication technologies
used to create the powder skeleton can create internal feeder
channels 360, 362, 364, 366 to carry the liquid to remote areas of
the skeleton 470. Such channels are considerably larger diameter
(by a factor of 5 or more, preferably between 5 and 10) than the
pore sizes and allow the liquid infiltrant to flow through the
feeder channels quickly without freezing. Indeed, for some SFF
processes, the size of such channels would need to be at least
three times larger than the powder diameter to facilitate powder
removal during fabrication of the skeleton. A network of such
channels can be designed into a part of complex geometry and
function as major arteries to supply liquid infiltrant to the
extremities. A relatively simple example is shown in FIG. 14, but
the channel geometry could be much more sophisticated if necessary.
The feeder channels can have a uniform cross section 362, or
varying 360 (for instance being larger nearer to the infiltrant
supply contact surface than farther from it). The feeder channels
can be vertical, horizontal, inclined, interconnected, or
independent.
In the case of relying on capillary forces to fill the feeder
channels, their size must be small enough to reach sufficient rise
height, given by the following equation [Fay]: ##EQU6##
with the definition of variables from equations 1 and 2, and r is
the radius of the channel. For a typical liquid metal surface
tension of 1 N/m, a 1 mm diameter channel would provide 4 kPa
capillary pressure. For liquid Ni of density 8 g/cc this would
correspond to a rise height of 5 cm. Channels 360 can be made with
variable diameter, starting larger at the bottom and decreasing in
size at the top to facilitate greater capillary rise. Note that
this would be a limitation on the height, but not on distance;
horizontal sections 366 would result in no loss of head.
Feeder channels can prove useful for overcoming a short penetration
distance limit when small powder, such as 20 micron, is used. Small
powder is more likely to have a short penetration distance limited
by freeze-off of the infiltrant. For instance, if the penetration
limit for a skeleton of 20 micron powder were only 2 cm, a network
of internal feeder channels can be designed into a 5 or 10 cm part
such that no section is more than 2 cm from a feeder channel
supplying liquid infiltrant. In general, feeder channels can be
arranged so that no region of the skeleton is spaced from a feeder
channel more than the penetration limit. Infiltrant will pass from
the feeder channels to the body of the skeleton through the walls
of the feeder channels along essentially their entire length. The
composition in the solidified feeder channels would match that of
the infiltrant rather than the bulk composition of the homogenized
part.
Skeletons with Fine Surface Texture Relative to Interior
A large penetration distance may be attained together with good
surface finish in another manner shown schematically in FIG. 15.
First, create a skeleton 570 out of a powder 530 that is large
enough to allow infiltration up to the desired height without
choking off the pore space. Next, apply a paste 520 of fine
metallic powder 522 with a particle size significantly smaller than
the size of the particles 530 constituting the skeleton 570. The
paste is then applied to the surface of the skeleton to create an
outer layer that has a surface finish superior to the original
skeleton, this outer layer is referred to as the covering layer.
The paste may be made with polymeric vehicles as thickeners and
binders and may be formulated to have a solids loading of typically
20-50% by volume metal powder. The skeleton 570 with the paste
applied is then fired to burn out any polymer in the paste 520 and
to sinter the fine powders in place. The skeleton with fine outer
covering layer is now infiltrated according to the manner of this
invention described above. The liquid infiltrant penetrates through
the main body of the skeleton traveling rapidly through the large
particle core. The infiltration slows appreciably as the infiltrant
reaches the covering layer 520 of the fine powder paste. However,
the infiltrant will only have to penetrate a small distance through
this layer of fine material and thus it will not choke off due to
diffusional solidification. The thickness of the covering layer
must be less than the penetration distance limit due to diffusional
solidification, but this constraint is easily satisfied because the
typical layer thickness is less than one diameter of the larger
particles. The fine powder could also be applied during the
fabrication of the part by selective deposition of slurry during
the SFF process. Such slurry deposition processes are described in
PCT/US98/12280, JETTING LAYERS OF POWDER AND THE FORMATION OF FINE
POWDER BEDS THEREBY, filed Jun. 12, 1998, published Dec. 17, 1998,
which is incorporated fully herein by reference.
The size of the powder 522 in the paste 520 should be between
approximately 1/100 to 1/10 the size of the powder 530 in the main
body of the skeleton. Thus, if 200 micron powder is used for the
skeleton, the paste should contain particles in the size range of
2-20 microns. The particles in the paste may be all of
approximately one size, or might span a range of sizes.
FIG. 15 shows two approaches to application of the paste 520 to the
skeleton 570. 1) The paste may be applied to the surface of the
skeleton to create a skin 524 of finer powder over the surface (as
shown on the left side of figure). 2) The paste can be designed to
penetrate into the pore spaces 526 and to smooth the surface by
filling in the space between the larger powder particles 530, but
not result in a layer on top of the larger particles (as shown on
the right side of figure). The second approach 526 has the
advantage of accurately maintaining the geometry of the original
component. However, if the composition of the fine powder in the
paste is the same as that of the large powder, the composition of
this region after infiltration and homogenization will be different
than that of the interior of the skeleton. This is because the
packing density of the larger powder with the additional fine
powder in the pore will exceed that of the original skeleton. This
is an advantage of the first approach, in that the packing density
of the applied layer will be approximately the same as that of the
bulk of the skeleton. However, the final composition using the
second approach can be made the same by using a fine powder in the
paste that has a composition different from that of the large
powder. The composition of the finer powder would actually have to
match that of the infiltrant, but some combination of the two
approaches (skin over the surface and penetration of the paste)
along with a carefully selected fine powder composition could
provide a desirable result. It may also be desirable to alter the
properties of the part near the surface, which could be done
through appropriate material selection for the fine powder. For
instance, high surface hardness can minimize wear due to friction
and a material with higher hardness could be selected for the fine
powder.
Maintaining Part Shape
Since infiltration is accomplished at temperatures close to the
melting point or solidus temperature of the skeleton, the
mechanical strength of the skeleton at the infiltration temperature
might be very low. Part distortion has been encountered when
suspending odd shaped parts above the melt. Distortion can happen
during the high temperature sintering, prior to infiltration. A
first step in minimizing part distortion can be achieved either
through changing the shape of the part or by supporting the part
from beneath rather than suspending it. FIGS. 16A and 16B show how
a large part that underwent distortion while hanging (16A) (note
holes for suspension support) experienced little or no distortion
while resting on the floor of a crucible (16B). For intricate part
shapes, simple floor support may not suffice. A loose ceramic
powder can be filled around the metal part to support parts with
intricate geometry. The infiltration can occur even while the part
is embedded in ceramic, because the ceramic powder is typically not
wet by the infiltrant, and thus the infiltrant will not enter those
regions.
Material Systems
Selection of appropriate material systems involves the choice of
skeleton material and MPD, with consideration for the degree of
infiltrant melting temperature depression, diffusivity and
solubility of the MPD in the skeleton material, and the desired
final material composition.
The inventors have conducted extensive experimental work involving
the binary Ni--Si material system, using a skeleton material of
pure nickel and an infiltrant of .about.90% Ni with the addition of
.about.10% Si. The specific amount of silicon used depends on the
infiltration temperature. Additions of other alloying elements to
this binary system can provide different, and for some
applications, more desirable mechanical properties. Such possible
alloying elements include, but are not limited to Chromium (Cr),
Iron (Fe), Cobalt (Co), and Molybdenum (Mo). Several commercial
alloys such as Inconel 617, HX, and G3 contain a combination of
those alloying elements along with 1% Si. For example, chromium
added to the Ni--Si system acts as a solid solution strengthening
element. (Commercial nickel brazing alloys containing silicon
typically contain 20% chromium for this reason.)
Other possible melting point depressants for nickel-based alloys
include boron (B), phosphorous (P), and tin (Sn). Boron and
phosphorous are used extensively in commercial brazing alloys. They
both have very low solubility, and would result in a two-phase
final part composition. Tin has a fairly high solubility that would
enable homogenization. Antimony (Sb) and sulfur (S) also provide
deep eutectics with nickel. Addition of large quantities of copper
(Cu) can significantly depress the melting point of nickel. As an
extreme case, a nickel skeleton infiltrated with pure copper would
also undergo diffusional solidification, due to the complete
solubility of the two elements with each other.
Aluminum (Al) offers many potential melting point depressants.
Table 1 summarizes the effect of several alloying elements commonly
used in aluminum. Pure aluminum has a melting point temperature of
.about.660.degree. C. Copper and magnesium (Mg) are typically used
to provide strengthening at small concentrations. Silicon is used
extensively in die casting alloys to improve fluidity of the melt.
Marching existing commercial die-casting alloy concentrations would
be useful, and can be done as an aspect of an invention disclosed
herein. Ternary and quaternary alloys can provide additional
melting point depression. For example, an aluminum alloy commonly
used in die casting of automotive pistons (336.0) contains
12Si-2.5Ni-1Mg-1Cu, has a solidus of 540.degree. C. and a liquidus
of 565.degree. C.
TABLE 1 Effect of various alloying elements on the melting point of
aluminum. Alloying Element Eutectic Melting Point Melting point in
Aluminum Comp (wt %) (.degree. C.) depression (.degree. C.) Silicon
(Si) 12 577 83 Magnesium (Mg) 35 450 210 Copper (Cu) 30 548 112
Germanium (Ge) 50 420 240 Lithium (Li) 8 596 64
One challenge for aluminum is to conduct the transient liquid-phase
infiltration within a small temperature window, since the melting
point depression of the infiltrant may be less than 100.degree. C.
Fortunately, the lower operating temperature of aluminum allows for
easier manipulation of the part and the melt.
The diffusivity of silicon in aluminum is .about.10.sup.-12 m.sup.2
/s at 600.degree. C., which is about one order of magnitude higher
than that of silicon in nickel at 1200.degree. C. The diffusion
distance is only affected by the square root of the diffusivity,
but this still presents more of a challenge in achieving large
infiltration depth as compared to a Ni--Si system. Factors such as
grain size and the presence of other species can influence the
diffusivity. Addition of iron to the infiltrant may be used to slow
mass transport, because of a high affinity of iron for ordering
with silicon and a lack of solubility of iron in aluminum. Copper
acts as an excellent barrier to Si diffusion and can be
electroplated on aluminum powder. The diffusivity of Cu in Al is
similar to that of Si, so the coating will nor last long at the
infiltration temperature, but a higher concentration of Cu at the
surface of the powder could still slow the mass transport
appreciably. The lower solubility of silicon in aluminum (as
compared to in Ni) will typically result in the liquid flow never
being choked off due to solidification. This is because the part
will only undergo partial diffusional solidification at the
infiltration temperature if the MPD final hulk composition is
greater than the MPD solidus composition.
Another challenge of processing aluminum alloys derives from the
natural formation of a thin surface layer of aluminum oxide,
potentially preventing wetting of the infiltrant, and having other
detrimental effects. The oxide grows faster at higher temperature.
Thus, minimizing the time the skeleton is exposed to elevated
temperature is beneficial and can be done through fast temperature
ramp rates and short dwell times. The furnace atmosphere can also
be controlled to slow the oxidation process. Using flux can also
help break down the oxide layer. Specific flux materials include,
but are nor limited to boric acid or others commonly used in the
aluminum welding and soldering industries. Adding small amounts of
magnesium also has a beneficial effect at breaking up the surface
layers of aluminum oxide. Using detergents or wetting aids to allow
the molten infiltrant to wet the oxide layer would also facilitate
infiltration.
It should be understood that in the claims appended hereto, if the
transitional phrase "consisting essentially" is used, the inventors
intend the claim to read on a composition that has the materials
identified in the claim and also small amounts of flux, detergent,
wetting agent, or magnesium, or other similar materials, which
small amounts do not adversely affect the depression of the melting
point.
An approach for infiltrating steel skeletons Involves using
multiple alloying elements to achieve the melting point depression
of the infiltrant. The ternary, quaternary, and greater complexity
alloys can provide significantly more depression of the melting
point than is achieved through any of the individual binary alloys.
Further, the concentration of alloying elements in the infiltrant
can be more than double that of the desired final composition,
because the infiltrant fills less than half of the total part
volume.
Table 2 below shows the melting range (characterized by the
liquidus and solidus temperatures) and composition of two common
stainless steels, 316 and 17-4 PH, along with the melting range of
an infiltrant that may be used to reach that standard
composition.
TABLE 2 Melting ranges of 316 and 17-4 PH stainless steels and
potential infiltrants. Liquidus Solidus C Mn Si Cr Ni Mo Cu Nb
Material (.degree. C.) (.degree. C.) (%) (%) (%) (%) (%) (%) (%)
(%) 316 1400 1339 0.08 2 1 17 12 2.5 Infiltrant 1292 1135 0.2 5 2.5
17 30 6.25 17-4 PH 1406 1237 0.07 1 1 16.5 4 4 0.3 Infiltrant 1298
1205 0.175 2.5 2.5 16.5 10 10 0.75
For these cases, the skeleton is composed of pure iron with a
melting point of 1538.degree. C. or iron and chromium with a
similar melting point. The infiltrant contains all of the necessary
alloying elements for the final composition so match that of the
standard stainless steel. For a 60% dense skeleton, this requires
the contribution of each alloying element to the infiltrant
composition to be 2.5 times greater than the desired final content.
Chromium does not have a significant impact on the melting point.
Thus, its concentration can be kept the same in the skeleton and in
the infiltrant. The processing window for the infiltration is over
200.degree. C., and the diffusivities of Ni, Mn, and Cu in iron at
1300.degree. C. are all approximately 10.sup.-14 m.sup.2 /s, which
is slow enough to allow infiltration before freezing. (The liquidus
and solidus information presented in Table 2 was calculated using
Thermo-Calc, a Computational Thermodynamics program used to perform
calculations of thermodynamic properties of multi-component systems
based on the Kaufman binary thermodynamic database.)
The infiltrant liquidus temperature dictates the minimum
infiltration temperature. In the case of 316 stainless steel, this
infiltration temperature (1292.degree. C.) lies below that of the
bulk material solidus (1339.degree. C.). This means that the
material will undergo complete diffusional solidification at the
infiltration temperature and the liquid flow will be choked off in
a rime period determined by the solidification rate. If the
infiltrant liquidus is above that of the bulk material solidus, as
is the case with 17-4PH steel, then the final part composition will
lie in a two-phase field and liquid will always be present at the
infiltration temperature. These two conditions are analogous to the
previous distinction made between skeleton material systems of low
and high solubility of MPD.
Titanium alloys have important uses in high temperature
applications where high specific stiffness and strength are
required. The binary Ti--Si phase diagram shown in FIG. 17 shows
very similar characteristics to the Ni--Si system discussed in
detail above. Although the processing of Ti parts is more
challenging, the materials behave similarly. Other alloying
elements that are common in commercial Ti alloys include but are
not limited to Al, Sn, Zr, Mo, V, Cu and Cr. Copper has a fairly
significant impact on the melting point, reaching a eutectic
temperature of 1005.degree. C. at a composition of 45 wt %.
Chromium and zirconium also work as melting point depressants in
titanium, although to a lesser degree.
Copper-based material systems are also good candidates for the
infiltration of a higher melting temperature skeleton with a
similar material used as an infiltrant. Potential melting point
depressants chat can be found in cast copper alloys are Ag, Mg, Mn,
Si, Sn, and Ti.
Many techniques and aspects of the inventions have been described
herein. The person skilled in the art will understand that many of
these techniques can be used with other disclosed techniques, even
if they have not been described as being used together. Thus, the
fact that a subcombination of features that are described
separately, may not be described in subcombination, does not mean
chat the inventors do not regard any such subcombination as an
invention chat is disclosed herein.
For instance, any of the following techniques and features can be
used with any of rho others: skeleton with feeder channels;
skeleton with a relatively coarse inner powder, with surfaces
covered with a paste formed from relatively finer powder; liquid
infiltrant supply tabs to introduce liquid to the skeleton at
multiple locations; providing the infiltrant supply at a desired
liquidus composition to facilitate uniform bulk composition along
the path of infiltration; agitating the infiltrant supply to insure
that it remains at such a desired composition; using an infiltrant
with a melting point depressant that diffuses into the skeleton
material, thereby tending to homogenize the composition of the
finished part, in some cases (high solubility) completely, and in
other cases (lower solubility) to a degree similar to cast
products; choosing powder size to insure infiltration to the full
extents of the skeleton in consideration of a penetration distance
limit imposed due to diffusional solidification, with relatively
larger particle sizes allowing greater penetration distance before
freezing, and relatively smaller particle sizes having a greater
capillary rise limit in the absence of freezing; choosing powder
surface area (roughness) to achieve penetration to the desired
extent, with relatively rougher surface area particles providing
greaser capillary driving force, faster and thus, deeper
penetration than relatively smoother surface area particles, other
factors being equal; choosing material systems with MPD that will
diffuse within the skeleton material to a degree necessary to
achieve homogenization of bulk properties, and, if possible,
composition, but at a rare that is slow enough to permit full
infiltration of the skeleton before diffusional solidification (if
any) occurs to a degree sufficient to choke off flow of liquid
infiltrant into the skeleton. Any of these general principles and
techniques can be applied to any of the material systems disclosed,
or hereinafter developed.
Some of the inventions disclosed herein are methods of fabricating
metal parts. However, other inventions disclosed herein are methods
of designing processes for fabricating such metal parts. In other
words, the process design inventions are methods for designing
manufacturing processes. For instance, it is disclosed how a
designer, challenged with the task of fabricating a metal part of a
specified shape, and specified basic metal (e.g., a predominantly
nickel part, or a predominantly aluminum part) will proceed to
design the process to make the part. The disclosure herein teaches
how the designer shall select a powder composition including the
base metal and alloying elements, and also how to select an
infiltrant, composed of the metal of the powder, and melting point
depressant agents. (The skeleton metal can also include some
smaller amount of these MPD agents.) The disclosure also teaches
chat a relatively small powder size should be first considered if
smooth surface finish is desired, and then if such size is too
small to permit full infiltration due to penetration distance
limits, instead a larger particle size must be selected. The
designer is also informed by this disclosure of the effects of
particle size, surface roughness, density, viscosity, and myriad
other factors that can be considered in rho selection of the
materials of rho skeleton and MPD. Additional mechanical techniques
are disclosed to overcome, or minimize the effect of material based
infiltration penetration limits. These mechanical techniques
include but are not limited to: feeder channels, fluid supply tabs,
covering a relatively coarse skeleton with a paste of finer
particle, and using particles with a rough surface. Thus, the
designer is taught how to achieve Infiltration penetration distance
greater than would be achieved in a comparison system, without the
enhancement, for instance, feeder channels, rougher particle
surface, or relatively fine powder surrounding a skeleton of
relatively coarser powder.
Further, the disclosure teaches how to achieve various degrees of
uniformity in composition, including substantially fully
homogeneous, homogeneous along the direction of infiltration, and
non-homogeneous, but similar in microstructure to cast products,
resulting in essentially homogeneous properties. These teachings
are based on maintaining the infiltrant at a liquidus composition,
and more subtle selection criteria related to the ratios of
components in the infiltrant as diffusional solidification takes
place in ternary and higher infiltrant systems, facilitated by
resort to equilibrium phase diagrams. All of these tools relate to
the inventions of designing a process of fabricating a metal
part.
This disclosure describes and discloses more than one invention.
The inventions are set forth in the claims of this and related
documents, not only as filed, but also as developed during
prosecution of any patent application based on this disclosure. The
inventors intend to claim the various inventions to the limits
permitted by the prior art, as it is subsequently determined to be.
No feature described herein is essential to each invention
disclosed herein. Thus, the inventors intend that no features
described herein, but not claimed in any particular claim of any
patent based on this disclosure, should be incorporated into any
such claim.
An abstract is submitted herewith. It is emphasized that this
abstract is being provided to comply with the rule requiring an
abstract that will allow examiners and other searchers to quickly
ascertain the subject matter of the technical disclosure. It is
submitted with the understanding that it will not be used to
interpret or limit the scope or meaning of the claims, as promised
by the Patent Office's rule.
This is being filed of even date with a Patent Application under
the Patent Cooperation Treaty, designating The United States of
America, in the names of the same inventors (Sachs, Lorenz and
Allen) , entitled INFILTRATION OF A NET SHAPE POWDER METAL SKELETON
BY A SIMILAR ALLOY WITH MELTING POINT DEPRESSED TO CREATE A
HOMOGENEOUS FINAL PART, Attorney Docket No. MIT 8873 PCT, being
filed under Ex. Mail Label No. EL662947541US the full disclosure,
of which is incorporated fully herein by reference, including the
specification, claims and figures.
The foregoing discussion should be understood as illustrative and
should not be considered to be limiting in any sense. While the
inventions have been particularly shown and described with
references to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the inventions as defined by the claims.
The corresponding structures, materials, acts and equivalents of
all means or step plus function elements in the claims below are
intended to include any structure, material, or acts for performing
The functions in combination with other claimed elements as
specifically claimed.
* * * * *